In all relationships, remember Matthew 5:17-18. Moses is still the reference in all matters of faith and sex (Leviticus 18) for the believers.

Matthew 5:17-18 Darby
(17) Think not that I am come to make void the law or the prophets; I am not come to make void, but to fulfil.
(18) For verily I say unto you, Until the heaven and the earth pass away, one iota or one tittle shall in no wise pass from the law till all come to pass.

There are sexual relationships in the Jewish Bible before and after Moses. Some sexual relationships that were approved before Moses are prohibited after Moses. If the Jewish Bible does not condemn, rebuke, or prohibit a kind of relationship in the Jewish Bible, it is permissible in all cultures (taboo or not) based on the principle that it is easier for our world to disappear than one iota in the Law to change. In going to settle in Israel, Joshua reminded the Israelites that they had to follow the Law of Moses to the letter in order to behave wisely (Joshua 1:7). Luke 16:17 makes Joshua 1:7 still applicable forever. If Jesus condemned a relationship in Moses, that relationship is forever forbidden. If Jesus approved a relationship in Moses, that relationship is forever permissible whether the relationship is taboo or not in our culture.

In all relationships, remember Matthew 5:17-18. Moses is still the reference in all matters of faith and sex (Leviticus 18) for the believers.

Matthew 5:17-18 Darby (17) Think not that I am come to make void the law or the prophets; I am not come to make void, but to fulfil. (18) For verily I say unto you, Until the heaven and the earth pass away, one iota or one tittle shall in no wise pass from the law till all come to pass.

Matthew 5:17-18 Darby
(17) Think not that I am come to make void the law or the prophets; I am not come to make void, but to fulfil.
(18) For verily I say unto you, Until the heaven and the earth pass away, one iota or one tittle shall in no wise pass from the law till all come to pass.